Research Article Predicting Accumulation of Intermediate Compounds in Nitrification and Autotrophic Denitrification Processes: A Chemical Approach José Luis Campos , 1 Jacques Dumais, 1 Juan Pablo Pavissich, 1,2 Oscar Franchi, 1 Dafne Crutchik, 1 Marisol Belmonte , 3 Mart-n Faúndez, 4 Lorena Jorquera, 5 Alba Pedrouso , 6 Anuska Mosquera-Corral, 6 and Ángeles Val del R-o 6 1 Facultad de Ingenier´ ıa y Ciencias, Universidad Adolfo Ib´ a˜ nez, Avda. Padre Hurtado 750, Vi˜ na del Mar, Chile 2 Center of Applied Ecology and Sustainability (CAPES), Santiago, Chile 3 Department of Environment, Faculty of Engineering, University of Playa Ancha, Avenida Leopoldo Carvallo 270, 2340000 Valpara´ ıso, Chile 4 Chemical and Environmental Engineering Department, Technical University Federico Santa Mar´ ıa, Ave. Espa˜ na 1680, Valpara´ ıso, Chile 5 Escuela de Ingenier´ ıa en Construcci´ on, Facultad de Ingenier´ ıa, Pontifcia Universidad Cat´ olica de Valpara´ ıso, Avenida Brasil 2147, Valpara´ ıso, Chile 6 Department of Chemical Engineering, Institute of Technology, Universidade de Santiago de Compostela, E-15705 Santiago de Compostela, Spain Correspondence should be addressed to Jos´ e Luis Campos; jluis.campos@uai.cl Received 16 January 2019; Revised 26 May 2019; Accepted 24 June 2019; Published 7 July 2019 Academic Editor: Raf Dewil Copyright © 2019 Jos´ e Luis Campos et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Nitrifcation and sulfur-based autotrophic denitrifcation processes can be used to remove ammonia from wastewater in an economical way. However, under certain operational conditions, these processes accumulate intermediate compounds, such as elemental sulphur, nitrite, and nitrous oxide, that are noxious for the environment. In order to predict the generation of these compounds, an analysis based on the Gibbs free energy of the possible reactions and on the oxidative capacity of the bulk liquid was done on case study systems. Results indicate that the Gibbs free energy is not a useful parameter to predict the generation of intermediate products in nitrifcation and autotrophic denitrifcation processes. Nevertheless, we show that the specifc productions of nitrous oxide during nitrifcation, and of elemental sulphur and nitrite during autotrophic denitrifcation, are well related to the oxidative capacity of the bulk liquid. 1. Introduction Removal of reduced nitrogen species from wastewater is conventionally carried out by means of nitrifcation and denitrifcation biological processes, where ammonia nitro- gen (NH 4 + ) is frst converted to nitrate (NO 3 − ) and then to nitrogen gas (N 2 ) [1]. Wastewater nitrifcation occurs under aerobic conditions whereas denitrifcation is anaero- bic. Nitrifcation is conducted in two consecutive steps by nitrifying microorganisms: ammonia conversion into nitrite by ammonia oxidizers (i.e., partial nitrifcation) and then nitrite conversion into nitrate by nitrite oxidizers. Te most studied nitrifers are ammonia-oxidizing bacteria (AOB), such as Nitrosomonas, and nitrite-oxidizing bacteria (NOB), where Nitrobacter is the most referenced genus. Nitrifers are chemolithotrophic and their activity decreases as the pH is reduced below neutrality [2]. Typically, denitrifcation relies on the oxidation of organic carbon by heterotrophic bacteria, and readily biodegradable carbon sources such as methanol, ethanol, and acetate, Hindawi BioMed Research International Volume 2019, Article ID 2051986, 9 pages https://doi.org/10.1155/2019/2051986